As types of bridges in which cables made of steel or such support girders, suspension bridges and cable-stayed bridges are widely known. In a suspension bridge, hanger ropes are vertically hung down at intervals from a main cable that spans bridge towers and then the hanger ropes support a girder of the bridge.
Suspension bridges are advantageous in embodying bridges of great length. Thus many bridges of great length crossing over straights or great rivers are constructed as suspension bridges. To bear great weight of very long girders, a number of steel wires are densely bundled together to form a strand and a number of such strands are further bundled together to form a main cable thereof in general. In between adjacent strands, and further in between adjacent steel wires, a great number of very narrow gaps exist. If rainwater intrudes into these narrow gaps, the rainwater accumulates over a long duration and therefore causes corrosion of the cable. A main cable, after once being laid, is actually irreplaceable and therefore degradation by corrosion is a very significant problem. More specifically, in regard to a lifetime of a bridge of great length, prevention of rusting of its cable is an important technical problem.
Japanese Patent Application Laid-open No. H08-177012 discloses an art for preventing rusting of a main cable, in which strands are respectively enclosed in seal pipes respectively ventilated with dried air.